Restore "-batch" original behavior, fix #57

From my understanding, '-batch' is intented to "disable interactive mode",
by closing stdin and thus ensure the process doesn't hang with a shell
prompt.

For #42, f8a021375d6d6cfc38c9ce05816785344d9373a8 changed '-batch'
meaning to 'background mode', which is in my opinion wrong. Actually the
problem reported in #42 is not well phrased since "& background" startup does
not work either with or without '-batch' mode enabled.

For these reason, this commit:
- Revert f8a021375d6d6cfc38c9ce05816785344d9373a8
- Reintroduce 'shell' without argument, even in batch mode
  (it does not hurt, and even demonstrate that batch mode works fine)
  See 141980a1bb81f236f012276f4f51abe52f7813d8
- Revert 3037954f7e23b8f5d48b28c46ff2b11b7afa40fa since 'batch mode'
  shoudn't mean 'background mode'